(1) Field of the Invention
The present invention relates to a method and apparatus for continuously and evenly drawing off samples of gases which are contained in a liquid laden with solids. More particularly, the present invention relates to a method and apparatus for obtaining samples of gases contained in drilling mud from an oil well or oil exploration site.
(2) State of the Prior Art
A normal practice at oil exploration and drilling sites is to use a special fluid, termed "drilling mud," to circulate in a drilling well. This fluid, whether the fluid is water or oil based, brings to the well surface rocky debris produced by the drilling of the well by the drill bit. Gaseous products originally contained in the drilled rocks are held in suspension in the fluid bringing the rocky debris to the surface. The gaseous products suspended in the fluid may be composed of hydrocarbons, which are indications that oil or gas deposits exist at the drilling site.
It is conventional to use the special drilling fluid or drilling mud to bring to the surface rocky debris and traces of hydrocarbons so that samples of the hydrocarbons may be extracted and analyzed. By this analysis, the geologic succession of strata penetrated by the drill can be determined and the existing opportunities for mining gas or oil deposits can be deduced. This practice is termed "mud logging."
With present methods, rocky debris is separated out of the fluid by filtration on vibrating screens. The gas, i.e. hydrocarbon, samples are collected in a gas trap. This gas trap normally comprises a chamber open at its base and immersed in the flow of mud emerging from the well. The occluded gases in the mud flow rise in the chamber. The chamber is fitted with an overflow for discharge of the mud in the chamber. The gases are suctioned through the chamber by the centrifugal effect of an agitator disposed in the chamber, and are then released by a regulating nozzle into a conventional scanning system.
FIG. 1 schematically illustrates a conventional mud degassing apparatus as described above. This device has a chamber 1, the chamber 1 being unitary and one piece with a support (not shown). The chamber is positioned on the upper portion of a tank 2 into which drilling mud is poured through a feed tube or chute 3. The drilling mud fills the tank 2 up to a level indicated by numeral 4 and is discharged by an overflow device 5 onto a vibrating screen 6 positioned above an outlet 7. The chamber 1, which may be cylindrical, has an inner shaped collar 8 at its lower end defining a circular opening 9. An agitator bar 11 is carried by a shaft 12 of a drive motor 13 and is positioned above the circular opening 9. A mud drainage nozzle 10 is positioned on the side of the cylindrical chamber above the agitator bar 11. The degassing chamber 1 has on its upper end a nozzle 14 for evacuating the gases collected inside the degassing chamber.
With this conventional device, it is impossible to know the precise volume of mud subjected to degassing over a given period of time. Nor is it possible to determine any definite relationship between the volume of degassed mud and the volume of gas released, in view of the heterogeneous nature of the mud in the tank and the relatively long path the mud has traveled from the outlet of the drilling well, the gas content of the mud being at least partially and unevenly lost before the mud discharges gas into the chamber 1.
As has been indicated above, in this system the flow rate of the mud, which may be approximately 58 liters per minute, is not precisely known. The flow rate of the mud varies as a function of the density and viscosity of the mud, and especially as a function of the changes in the level of the mud in the tank in which the degassing chamber is immersed. Furthermore, the exact proportion of gases which are extracted from the mud is unknown. Also, the need to have a plane surface of mud available (see FIG. 1) requires the placement of the gas trap at a fairly considerable distance from the outlet of the drilling well, normally over the feed reservoir of the vibrating screens. The mud reaching this site thus has travelled some distance in the open air and has partially discharged its gases into the air. Flow turbulence of the mud in the conduit or chute feeding into the vibrating screens further assists the discharge of the gas into the air. Furthermore, conventional devices of this type can only provide approximate or some times even false data, because some heavier hydrocarbons can easily avoid analysis and be directly lost or underestimated.
Accordingly, measurements taken with the above described apparatus cannot be quantified, and are not comparable from one point in time to another nor from one drill site to another, nor even from one apparatus to another. Although the gaseous mixture which is obtained is measured and analyzed with precision, there is no known stable relationship between the results obtained and the quantities which are actually present in the mud. Such a relationship would enable the deduction, by simple calculation, of the proportions and quantities of hydrocarbons contained in the respective geologic layers.
The exact measurement and analysis of gases conveyed by drilling mud are, therefore, of extreme importance in carrying out mud logging procedures. Furthermore, mud logging is the only means of gaining direct knowledge of the presence of usable hydrocarbons in the subterranean layers through which the drill bit penetrates, as well as the relative proportions of the various hydrocarbons detected, i.e. methane, ethane, propane, butane and pentane. It is the only manner in which an accurate reading can be obtained.
Although various devices have been used to determine the quantities of gases in drilling mud, none of these devices provides a continuous and automatic indication of the quantities of occluded gases in a given volume of mud corresponding to an easily identifiable geological layer.